Magnetic resonance imaging systems take advantage of magnetic resonance phenomena to obtain tomographic images of the interior of living subjects. This capability of obtaining tomographic images without using X-rays or other forms of radiation is advantageous because it removes the radiation risks. In addition magnetic resonance systems can provide excellent images of soft tissues as well as bone. Further MRI systems have the capability of obtaining physiogical images in addition to anatomical images.
MRI relies on the fact that certain isotopes, those having an odd number or neutrons or electrons exhibit magnetic moments that when subjected to a large static magnetic field tend to statistically align with the magnetic field. The aligned isotopes ("spins") rotate or precess around the axis of the large magnetic field at a rotational frequency that is equal to the gyromagnetic constant (.gamma.) for the aligned isotope multiplied by the strength of the magnetic field (B) applied to the isotope being imaged. Mathematically, then: .omega..sub.o =.gamma.B. This rotational frequency is known as the Larmor frequency.
If a patient is positioned in a strong static magnetic field and a radio frequency (RF) pulse having the Larmor frequency is applied to a portion of the patient, then the spins at that portion of the patient are "tipped" or perturbed into a transverse plane relative to the axis of the strong static magnetic field. The spins so tipped tend to dephase and revert to the original aligned position after removal of the RF pulse. The rotation in the transverse plane generates small signals that decay due to the dephasing and the reverting. These small signals are known as free induction decay (FID) signals since they occur after the removal of the radio frequency pulse. "Echo" signals are also generated responsive to steps taken to rephase the dephasing spins, i.e. Hahn echoes or gradient echoes. The time-to-echo (TE) is equal to the time between a "tipping" radio frequency pulse and the echo signal; which at a minimum is equal to twice the time tau (.tau.) between the "tipping" pulse and a rephasing pulse. The time-to-echo can be manipulated by the operator, but usually the time-to-echo should be kept at a minimum, especially for the first echo, to obtain maximum signals.
The location of the signals are determined using magnetic gradients. The magnetic gradients set the strength of the static magnetic field as a function of location. The frequency of the received signal is related to the strength of the magnetic field at the spins generating the signals and consequently to the location of the signal source. Thus the location of the source of the signals (FID or echo) can be determined and spatial images can be constructed.
Multi-dimensional images are acquired using encoding gradients along the axes of the transverse plane. In two-dimensional imaging one axis is time encoded and the other axis is phase encoded, for example.
It is to be understood that the acquired signals are exceedingly small and consequently anything that can be done to improve the signal-to-noise ratio (SNR) of the received signals is beneficial and important. Usually any improvement in the SNR comes at a cost of either time or resolution. For example, one method of improving the SNR is by repeating the acquisition of the signals and then averaging the repeated signals so acquired.
Averaging, of course, requires additional time for acquiring the duplicate or multiple signals and accordingly time is being paid for the improved SNR obtained thereby. Those skilled in the art are continuously searching for apparatus and methods for improving the SNR of the received signals in MRI systems. They are especially attempting to provide methods and apparatus for improving the SNR while expending a minimum amount of time or resolution.